1. Field of the Invention
The present invention relates to fiber optic communication transmission, and more particularly to a modulation format for fiber optic transmission having a low sensitivity to fiber non-linearity.
2. Description of the Related Art
Since the advent of fiber amplifiers (i.e. fiber optic line amplifiers), there has been an ongoing debate regarding the role of time-division-multiplexing (TDM) versus wavelength-division-multiplexing (WDM). Soon after the discovery of fiber amplifiers it became obvious that TDM alone would not be suitable to exploit the full amplifier bandwidth as it would require the use of femtosecond pulses. Thus, a mixture of TDM and WDM would be required. However, the question of which base bit rate to use for WDM still remained. There are many reasons to promote or reject the use of higher base bit rates. An argument against high-bit-rate based systems is that non-linear transmission is, particularly difficult and, in general, the technology has not matured to that level. With low-bit-rate based systems, one quickly realizes that the number of components (transmitters, receivers, add/drop (de-) multiplexers, etc.) increases to an unacceptable level, especially for future wide-band high-capacity systems. In the long run, however, with ever improving technologies, the most relevant consideration when choosing a bit rate may be related to the spectral efficiency associated with the base bit rate.
The spectral efficiency is dependent on system parameters such as the distance of the transmission, the type of optical fiber used and the amplifier spacing. The spectral efficiency is defined as the bit rate density in the spectral domain. Early WDM systems were based on 2.5 Gb/s channels spaced by 100 GHz. The spectral efficiency of such systems is 0.025 bits/Hz. For intensity-modulated formats, the ultimate spectral efficiency (in the absence of any propagation) can be estimated from the spectral overlap of adjacent channels. For Non Return-to-Zero Modulation (NRZ), spectral overlap limits the spectral efficiency to a value around 1 bit/Hz. This indicates that early WDM systems are far from close to the limit imposed by spectral overlap. The spectral efficiency of 2.5 Gb/s channels can be increased to 0.05 bits/Hz by adjusting the channel spacing to 50 GHz. Reducing the channel spacing, however, generally leads to increased power penalties after transmission from nonlinear interactions between channels.
The two main non-linear interactions specific to WDM systems are four-wave mixing (FWM) and cross-phase modulation (XPM). In 2.5 Gb/s systems, one can double the spectral efficiency to obtain 0.1 bits/Hz by using a higher base bit rate of 10 Gb/s with 100 GHz channel spacing. For such spacing, nonlinear WDM interactions, in general, do not dramatically affect the transmission. Going to higher bit rates generally allows dramatic reduction of nonlinear WDM interactions but makes single-channel transmission increasingly difficult (i.e. transmission of a single channel in the absence of any other channel). For instance, while no dispersion compensation is required for single-channel transmission at 2.5 Gb/s, transmission of a 10 Gb/s signal over terrestrial system distances (i.e. distances up to about 640 km) requires careful and difficult to maintain dispersion compensation mapping accurate to a few hundredths of ps/nm at the end of the transmission line.
As mentioned above, nonlinear transmission at high bit rates (e.g.  greater than 10 Gb/s and more) is not limited by nonlinear interaction of WDM systems (many channels with different wavelength) as is the case for lower bit rates (i.e.  less than 10 Gb/s); the limitations are instead due to single channel effects. There are two types of single channel effects. The first type of single channel effect is the isolated pulse transmission (solitons are an example of a format designed around having the best isolated pulse transmission, whereas the second type of single channel effect is pulse-to-pulse interactions. Pulse-to-pulse interactions originate from the overlap between neighboring pulses after they have experienced some dispersion.
The use of pulses to counteract fiber non-linearity is an idea widely known and the concept of solitons and dispersion-compensated solitons (also known as dispersion-managed solitons, or DMS) has been applied in this context. It should be noted, that for solitons and DMSs, one attempts to reduce pulse spreading to minimize the overlap between adjacent pulses of the same channels.
In the effort to improve spectral efficiency, it becomes important to study bit rates exceeding 10 Gb/s. Specifically, and by way of example, 40 Gb/s is an important bit rate to consider, since by conventions of optical standards (i.e. SONET and SDH), 40 Gb/s is the next standard bit rate beyond 10 Gb/s.
The present invention provides a method for modulating fiber optic transmissions with low sensitivity to fiber non-linearity. The modulation format of the present invention uses either pulses shorter than those of solitons or DMSs, or uses a pulse width similar to solitons or DMSs but allows the pulses to spread well beyond what is considered soliton or DMS propagation. In addition, the modulation format takes advantage of spreading between adjacent pulses to average out nonlinear interactions between pulses.
In accordance with an embodiment of the present invention, the method utilizes short pulses (typically shorter than 20 ps) with broad spectral bandwidth, and bit rates of 10 Gb/s and higher to improve performance over heretofore studied and used nonlinear transmission with Return-to-Zero (RZ) format. The operable range for the short pulses can be as low as 0.004 ps up to 20 ps.
In another embodiment, pre- and/or post-dispersion compensation can be added to the transmission to further improve performance. Increasing of the bit rate above 40 Gb/s results in a broader spectral bandwidth which provides effects comparable to using shorter pulses at lower bit rates without requiring pre-dispersion compensation. For example, bit rates in excess of 100 Gb/s inherently have broader spectral bandwidth which also reduces the effect of non linearity in transmission, and may not require using very short pulses.
In one embodiment at which the base bit rate is 40 Gb/s, the modulation method of the present invention includes the steps of pre-dispersion compensating of the transmission information, transmitting the information through the fiber, and post-dispersion compensating the transmission information to obtain 100% cumulative compensation at the end of the transmission fiber.
The present invention is described, inter alia, in the form of a systematic study of nonlinear transmission of a single 40 Gb/s channel. It is observed that from the perspective of nonlinear single-channel transmission at 40 Gb/s, the NRZ format does not provide the optimum system performance. Although not optimum, NRZ format may, nonetheless, also be used. When transmitting at 40 Gb/s, significantly improved nonlinear transmission can be achieved by using the Return-to-Zero (RZ) format with a low duty cycle (e.g. xcx9c20%). The advantages obtained through use of a format with a low duty cycle increase with the dispersion of the transmission fiber.
When transmitting at 40 Gb/s, the step of pre-dispersion compensation is performed based on the average position of the point of zero cumulative dispersion inside each transmission fiber. The average point of zero cumulative dispersion is a distance at which the amount of pre-compensation is adjusted to provide accurate reconstruction of the transmitted signal at the receiving end. This average point of zero cumulative dispersion for a base bit rate of 40 Gb/s has been determined to be within a 0 km-20 km span of the transmission fiber. This distance range corresponds to roughly the effective length (length of fibers having 70% power loss) of the transmission fiber.
It is important to note that as the base bit rate is increased (i.e. above the 40 Gb/s shown here), thereby broadening the spectral bandwidth of the signal, the range for the average point of zero cumulative dispersion will increase and can result in a distance of 0 km or less indicating that no pre-dispersion compensation or even negative distance pre-compensation of the same sign as the transmission fiber is required.
The modulation technique described herein is not limited to format transmission and can be applied to any coherent digital or analog signal transmission having a broad bandwidth for transmitting and carrying information. Stated differently, the modulation technique applies to all signals for which the nonlinear length (i.e. length over which the signal phase distorted by the fiber non-linearity) at high power becomes shorter than the dispersion length (i.e. the length over which one sees significant signal evolution from dispersion).
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.